Field of the Invention
The present invention relates to high-voltage electrostatic particle accelerators.
Description of the Prior Art
A high-voltage electrostatic particle accelerator is described in XP-002665162 Proceedings of IPAC '10 Kyoto, Japan, pp. 711-713 P. Beasley, O. Heid, T. Hughes “A new life for High Voltage Electrostatic accelerators”.
An example of such an accelerator is shown in FIG. 1. In such accelerators, concentric conductive half-shells 10 are provided, electrically isolated from one another, but interconnected with diodes in a Cockroft-Walton (Greinacher) cascade. The concentric conductive shells provide the required capacitance. The shells may be enclosed within a vacuum vessel (not shown) such that the space around and between half-shells 10 is evacuated. Application of an AC voltage to the assembly causes each shell 10 to be charged to a certain DC voltage with respect to the next, resulting in a very large electrostatic potential difference between in the innermost and outermost shells.
FIG. 2 shows a simplified diagram illustrating the connection of diodes 15 between half-shells 10, and the connection of the AC supply 17. As shown, an AC supply 17 is connected between ground 30 and one half-shell, labelled 10a in the drawing. This is capacitively coupled to an inner adjacent half-shell labelled 10b. This is in turn capacitively coupled to an inner adjacent half-shell labelled 10c, and this is in turn capacitively coupled to an inner adjacent half-shell labelled 10d. 
The AC voltage is capacitively coupled between half-shells 10a-10d. These AC-coupled half-shells are connected by diodes 15 to corresponding DC half-shells 10e-10h in the drawing to form the required Cockroft-Walton (Greinacher) cascade. In operation, the applied AC voltage is rectified and multiplied by twice the number of sets of half-shells used, so the maximum voltage which accumulates on the innermost DC half-shell 10h is 2×4×Vac. Voltages in the megavolt range are usually obtained. The voltage Utotal at the innermost DC half-shell may be expressed as Utotal=2nU0, with a superimposed ripple voltage. U0 is the peak value of the AC input voltage Uin, so that Uin=U0 sin(ωt)
Four concentric pairs of half-shells 10 are shown in FIG. 2, but a high-voltage electrostatic generator of the type addressed by the present invention may have rather more, or fewer, concentric pairs of half-shells depending on the desired output voltage.
By providing a path for a particle beam through the DC half-shells 10e-10h, a compact high-voltage electrostatic particle accelerator may be constructed.
FIG. 3 schematically illustrates such an accelerator in part-cross-section. In FIG. 3, the structure is essentially cylindrically symmetrical about axis A-A, with the exception of aligned holes 19 which form a path for a particle beam through the DC half-shells 10e-10h. References herein to ‘radial’ or ‘axial’ directions are intended with respect to this axis. The accelerator comprises a series of pairs electrically conductive half-shells 10 one connected via the external AC drive and the other with the developed DC voltages. The half-shells of each pair are spaced apart by an equatorial gap 14. A significant DC potential difference accumulates between the concentric shells, with the outermost DC half-shell typically being at ground voltage, and the innermost half-shells typically being at several megavolts. Structural integrity of the accelerator is provided by solid electrical insulators (not shown) between half-shells.
Conventionally, as illustrated in part-cross-section in FIG. 3, such high voltage electrostatic generators have half-shells 10 which are parallel to one another right up to their edge regions 16 on either side of the gap 14. The separation distance s between adjacent half-shells may vary, and this may be useful in providing an appropriate voltage gradient for an accelerating particle at all positions along the beam path, as the particle speed increases. The half-shells 10 are typically made from thin conducting materials with a square or rounded edge profile. Typically, the electrostatic generator is designed to be as small and lightweight as possible. A significant contribution to both of these aims is provided by using thin metal sheet for the half-shells.
A series of aligned holes 19 in the DC coupled half-shells provides a path for beam acceleration.
FIG. 4 shows a magnified part of the electrostatic generator of FIG. 3. The vacuum chamber 12 is electrically conductive, and grounded. In this example, it is spaced from the half-shells 10 by a distance d greater than the separation s between any two adjacent half-shells, although this need not be the case.
In FIG. 4, the edge regions 16 of the half-shells 101-106, 111-116 are cut square or can be rounded, particularly indicated at 23 in the magnified view of the edge region 16 of half-shell 102. This is for manufacturing convenience, as it would be very difficult to put any other edge region profile on such a thin material. The attendant corners 23 give rise to regions of high electrostatic stresses, shown at 18, due to the resulting change in field lines close to the shell edge, even with rounded edges.
Lines of electrostatic equipotential are shown in the region of gap 14. Away from the equatorial gap 14, the lines of equipotential will run parallel to the adjacent half-shell(s), but are not shown in the drawing. A bunching of lines of electrostatic equipotential represents a relatively high value of electrostatic stress.
The high electrostatic stresses are most pronounced at the edge regions 16 of the outermost half-shells 106, 116, particularly near their inner surfaces. The next most pronounced high electrostatic stresses are at the edge regions 16 of the innermost half-shells 101, 111, particularly near their outer surfaces.
Regions of high electrostatic stress are to be avoided, and to be eliminated so far as is practicable. Regions of high electric stress may cause a breakdown in the isolation between half-shells, for example through vacuum or air. Such electrostatic discharge will cause damage to the material of the shells, and a loss of accumulated charge, meaning that a target voltage of the innermost DC-connected half-shell may not be reached. The sudden peaks in current associated with electrostatic discharge may damage the power supply and diodes associated with the electrostatic generator.
In pursuit of the aims of a small size and light weight, the electrostatic generator will typically be constructed with a minimum number of concentric shells. This will in turn mean that a relatively large potential difference arises between adjacent DC half-shells, tending to encourage electrostatic breakdown.
Although some rounding of the corners 23 has been employed in known arrangements, the high stress regions 18 have been found not significantly diminished by these efforts.